Could we consolidate data in faceted specs?

[joelostblom]

There are situations like the below that raises the error although the same data is technically used, just respecified as a new variable.

alt.layer(
    alt.Chart(data.movies.url).mark_rule().encode(x='IMDB_Rating:Q'),
    alt.Chart(data.movies.url).mark_rule().encode(x='IMDB_Rating:Q')
).facet(
    'Major_Genre:N'
)

I am not sure if altair should be capable of figuring out that this is the same as

source = data.movies.url
alt.layer(
    alt.Chart(source).mark_rule().encode(x='IMDB_Rating:Q'),
    alt.Chart(source).mark_rule().encode(x='IMDB_Rating:Q')
).facet(
    'Major_Genre:N'
)

and avoid the error, maybe those comparisons will be too complex for large data frames and non-portable across dataframe implementations (although many have some type of equality test built-in like pandas). But maybe we can explore if these datasets could be consolidated as is done for other data? See this function: https://github.com/altair-viz/altair/blob/main/altair/vegalite/v5/api.py#L63 cc @mattijn